My experimentation with WordPress plugins began in 2012. In 2016, I took over the development of Chamber Dashboard plugins.
Although there were a lot of times, I was ready to pull my hair out, I have fun working with these plugins. I also learned a lot along the way.
WP Category Dropdown
This is a simple plugin that displays parent and child categories in two dropdown. The child category dropdown is generated based on the selected parent category. Read more about the plugin here – https://www.gcsdesign.com/wp-category-dropdown/
The plugin can be downloaded from the WordPress plugins page – https://wordpress.org/plugins/wp-category-dropdown/